Method and Device for Converting Thermal Energy Into Mechanical Work

ABSTRACT

The invention relates to a method for converting thermal energy into mechanical work. Said method comprises the following steps which are performed as a cycle: A liquid work medium is fed from a supply reservoir ( 1 ) to a work container ( 3 ); the work medium in the work container ( 3 ) is heated by a first heat exchanger ( 5 ); a sub-amount of the work medium flows from the work container ( 3 ) to a pneumatic-hydraulic-converter ( 8 ), a hydraulic medium from the pneumatic-hydraulic-converter ( 8 ) is compressed in a work machine ( 9 ) in order to convert the hydraulic work of the hydraulic medium into mechanical work; the work medium from the pneumatic-hydraulic-converter ( 8 ) is fed back into the supply reservoir ( 1 ) and the hydraulic medium is returned into the pneumatic-hydraulic-converter ( 8 ). The invention also relates to a device for carrying out said method.

The present invention relates to a method and to an apparatus forconverting thermal energy into mechanical work.

Many kinds of cyclic processes and apparatus for converting thermalenergy into mechanical work and, where required, from there to electricpower are known. These processes are for example steam power processes,Sterling processes or the like. One possibility of utilizing suchmethods is to increase the efficiency of internal combustion engines bymaking use of the waste heat. The problem here however is that theavailable temperature levels are quite disadvantageous since the coolingcircuit of internal combustion engines usually operates at temperaturesof about 100° C. A similar problem arises when heat from solar powerplants is to be converted into mechanical work.

A special solution for such a thermal power process is shown in thedocument WO 03/081011 A. In this document, there is described a methodby which a hydraulic fluid is pressurized by heating a working fluid ina plurality of bladder accumulator means, said hydraulic fluid beingworked off in a working machine. Although such a method is working inprinciple, it has been found that its efficiency is moderate and that,compared to the amount of energy that can be generated, equipmentexpense is quite high.

A discontinuously operated method capable of generating work throughheat conversion at moderate efficiency is further known from U.S. Pat.No. 3,803,847 A.

It is the object of the present invention to configure a method of thetype mentioned herein above in such a manner that high efficiency isachievable even under thermally disadvantageous conditions, with theequipment expense being as low as possible.

In accordance with the invention, such a method consists of thefollowing steps, which are performed as a cyclic process:

-   -   supplying a liquid working fluid from a storage reservoir to a        work tank;    -   heating the working fluid in the work tank via a first heat        exchanger;    -   allowing a fraction of the working fluid from the work tank to        overflow into a pneumatic-hydraulic converter, this causing a        hydraulic fluid to be urged from the pneumatic-hydraulic        converter into a working machine for conversion of the hydraulic        work of the hydraulic fluid into mechanical work;    -   returning the working fluid from the pneumatic-hydraulic        converter into the storage reservoir by recirculating hydraulic        fluid into the pneumatic-hydraulic converter

In the first step, a working fluid having an appropriate vapor pressurecurve such as for example R134 a, that is 1,1,1,2-tetrafluoroethane, isdrawn from a storage reservoir. The working fluid in this storagereservoir is in an equilibrium state between a liquid phase and agaseous phase. The pressure is hereby chosen such that this equilibriumis maintained. In the case of R134 a and of an ambient temperature ofabout 20° C., this first-pressure will be about 6 bar. The working fluidis transferred to a work tank in which it is preferred that a second,higher pressure prevails. The second pressure is for example 40 bar. Theenergy expense for the transfer can be minimized if, in a preferredmanner, only liquid working fluid is transferred to the work tank bypumping. In the second step, the working fluid is heated in the worktank. Heating causes the pressure to increase even more and the workingfluid evaporates partially. Heating preferably occurs through wasteheat, for example from an internal combustion engine. If the workingfluid is heated to a temperature of 100° C., the waste heat can beoptimally utilized.

In the third step, the working fluid is allowed to overflow into apneumatic-hydraulic converter. This can occur after the second step,i.e., the heat is completely supplied first and the connection betweenthe work tank and the pneumatic-hydraulic converter is establishedthereafter. These steps may however also be performed in part or inwhole simultaneously, i.e., the fluid in the work tank is heated whileit is flowing into the pneumatic-hydraulic converter. In this way, theefficiency can be optimized since the cooling effected by the expansionof the working fluid is immediately accommodated. Moreover, the cycletime is shortened. In the pneumatic-hydraulic converter, which is forexample implemented as a bladder accumulator, the inflowing workingfluid displaces a hydraulic fluid that is present in the hydraulicchamber and is being worked off in a suited working machine, for examplea hydraulic motor, in order to produce mechanical work that may in turnbe used to produce electrical energy.

In the fourth step, the pneumatic-hydraulic converter is re-filled withhydraulic fluid through a small pump, with the working fluid beingdisplaced and recirculated into the storage reservoir. Whereappropriate, the working fluid is thereby directed through a second heatexchanger, this making it possible to adapt the temperature to theambient temperature.

After this fourth step, the cyclic process is continued with the firststep.

The efficiency and the performance of the system can be optimized if thepossible phase transitions are made use of accordingly. Morespecifically, in the first step, the working fluid should be moved inthe liquid state only, whereas in the third step, only the gaseous phasewill be transferred to the pneumatic-hydraulic converter.

Preferably, there is provided that during recirculation of the workingfluid from the pneumatic-hydraulic converter into the storage reservoirthe connection between the work tank and the pneumatic-hydraulicconverter is interrupted. This permits to minimize overflow losses.

The efficiency may be optimized if the working fluid is cooled whilebeing supplied from the storage reservoir to the work tank. Cooling canoccur through an ambient heat exchanger, meaning through a currentcooler, but it is also possible to use cold produced by the second heatexchanger provided it is not needed for some other purpose, for examplefor an air conditioning system or a cooling aggregate.

A particular effect of benefit is achieved if the hydraulic fluid iskept at a temperature that corresponds to the mean temperature of theworking fluid in the pneumatic-hydraulic converter. This way,undesirable temperature compensating effects can be avoided.

As already explained, it is possible that the working fluid be directedfrom the pneumatic-hydraulic converter through a second heat exchanger.Depending on the way of conducting the method, low temperaturesoccasioned by the expansion of the working fluid may be generated in thesecond heat exchanger. These low temperatures can be used for cooling inorder to economize the energy needed there.

Another improvement of the production of low temperatures can beachieved by causing the working fluid from the pneumatic-hydraulicconverter to expand to an expansion pressure that is lower than thefirst pressure in the storage reservoir and is next compressed to thefirst pressure.

The invention further relates to an apparatus for converting thermalenergy to mechanical work, said apparatus having a storage reservoir, awork tank and a working machine for converting hydraulic work intomechanical work.

In accordance with the invention, there is provided that the work tankis connected to a first heat exchanger for heating the working fluid,that the work tank is further connected to a pneumatic-hydraulicconverter that transfers the pressure of the working fluid to ahydraulic fluid and that there is provided a recirculation line forrecirculating the working fluid from the pneumatic-hydraulic converterinto the storage reservoir.

In a particularly preferred implementation variant, there is providedthat a plurality of work tanks and pneumatic-hydraulic converters areconnected in parallel.

In practical implementation, five of the apparatus illustrated in FIG. 1are for example arranged parallel to each other in a side-by-siderelationship and operated in a time-staggered fashion as this is forexample the case in a five-cylinder internal combustion engine. Thispermits to achieve continuous operation without noteworthy cyclicfluctuations.

The method of the invention and the apparatus of the invention will bediscussed in greater detail herein after with reference to the circuitdiagram of FIG. 1, which illustrates the major component parts of thesystem. FIG. 2 shows a typical vapor pressure curve of a working fluid.

A storage reservoir 1 holds a working fluid; a coolant such as R134 acan be utilized for example. The working fluid in the storage reservoir1 is in phase equilibrium at ambient temperature and at a pressure ofabout 6 bar. The storage reservoir 1 is connected to a work tank 3through a feed pump 2, this connection being switchable through a valve4. In the work tank 3 there is disposed a first heat exchanger 5 thatserves to heat the working fluid in the work tank 3. Heat exchanger 5 issupplied with waste heat from an internal combustion engine that has notbeen illustrated herein via a booster pump 6, with water at 100° C.being directed through the first heat exchanger 5 for example. Throughan overflow line 7, the work tank 5 communicates with a first workingchamber 8 a of a pneumatic-hydraulic converter 8 that is configured tobe a bladder accumulator. The first working chamber 8 a is separatedfrom a second working chamber 8 b by a flexible membrane 8 c thatseparates the two working chambers 8 a, 8 b while allowing for pressurecompensation. The second working chamber 8 b of the pneumatic-hydraulicconverter 8 communicates with a hydraulic circuit consisting of aworking machine 9 having a generator 10 flanged thereon, an oil tank 20,a recirculating pump 17 and a third heat exchanger 11. The third heatexchanger 11 is supplied from a pump 12. Another work line 19 connectsthe first working chamber 8 a of the pneumatic-hydraulic converter 8 toa second heat exchanger 16 that communicates through a booster pump 14with the storage reservoir 1. For the rest, the lines 7, 19 may beclosed selectively by valves 7 a, 19 a.

The mode of operation of the apparatus of the invention will beexplained in closer detail herein after:

In a first step, liquid working fluid is transferred from the storagereservoir 1 into the work tank 3 via the feed pump 2, with the pressurebeing increased from 6 bar to 40 bar.

After the work tank 3 is completely filled with liquid working fluid,the valve 4 is closed and heating through the first heat exchanger 5occurs. This heating constitutes the second step. Waste heat fromanother process can be used therefor.

By heating the working fluid to 100° C., part of said fluid evaporatesin the work tank 3 and this vapor is transferred in a third step,through the line 7 with the valve 7 a being open, into the first workingchamber 8 a of the pneumatic-hydraulic converter 8. The pressure drop iscompensated by further heating through the first heat exchanger 5.Simultaneously, the membrane 8 c of the pneumatic-hydraulic converter 8is displaced toward the second working chamber 8 b so that hydraulicfluid is urged through the working machine 9 driving the generator 10.The third step ends as soon as the second working chamber 8 b of thepneumatic-hydraulic converter 8 has largely emptied.

In a fourth step, hydraulic fluid is recirculated via the pump 17 fromthe tank 20 into the second working chamber 8 b of thepneumatic-hydraulic converter 8 and the working fluid is directed fromthe first working chamber 8 a, through the valve 19 a in the line 19,which has opened in the meantime, through the second heat exchanger 16and is expanded. A booster pump 14 recirculates the working fluid backinto the storage reservoir 1. As denoted by the arrow 21, the heatabsorbed by the working fluid in the second heat exchanger 16 can beevacuated as cooling capacity for operating a cooling system or an airconditioning system. A partial flow through a heat exchanger 15 may alsobe used for cooling the working fluid during compression, though.

FIG. 2 illustrates a typical vapor pressure curve of a working fluidadapted for use in the cyclic process described herein above. Saidworking fluid is R134 a, which is known to be a coolant, meaning1,1,1,2-tetrafluoroethane. As can be seen, at ambient temperature and ata pressure of about 6 bar, the liquid phase is in equilibrium with thegaseous phase. At a temperature of 100° C., this equilibrium pressure isabout 40 bar.

With simple equipment structure the present invention allows for optimaluse of waste heat from other processes, like for example from theoperation of an internal combustion engine.

1. A method for converting thermal energy into mechanical work, saidmethod involving the following steps that are performed as a cyclicprocess: supplying a liquid working fluid from a storage reservoir (1)to a work tank (3); heating the working fluid in the work tank (3) via afirst heat exchanger (5); allowing a fraction of the working fluid fromthe work tank (3) to overflow into a pneumatic-hydraulic converter (8),this causing a hydraulic fluid to be urged from the pneumatic-hydraulicconverter (8) into a working machine (9) for conversion of the hydraulicwork of the hydraulic fluid into mechanical work; and returning theworking fluid from the pneumatic-hydraulic converter (8) into thestorage reservoir (1) by recirculating hydraulic fluid into thepneumatic-hydraulic converter (8).
 2. The method as set forth in claim1, comprising compressing the working fluid from a first, lower pressurein the storage reservoir (1) to a second, higher pressure in the worktank (3).
 3. The method as set forth in claim 2, comprising transferringthe working fluid in a liquid form from the storage reservoir (1) to thework tank (3).
 4. The method as set forth in claim 3, wherein theworking fluid evaporates partially while being heated in the work tank(3) and is directed in the gaseous state from the work tank (3) into thepneumatic-hydraulic converter (8).
 5. The method as set forth in claim4, comprising heating isochorically the working fluid in the work tank(3).
 6. The method as set forth in claim 5, wherein the connectionbetween the work tank (3) and the pneumatic-hydraulic converter (8) isinterrupted by a valve (7 a) while the working fluid is being returnedfrom the pneumatic-hydraulic converter (8) into the storage reservoir(1).
 7. The method as set forth in claim 6, comprising cooling theworking fluid by a heat exchanger (15) while it is being supplied fromthe storage reservoir (1) into the work tank (3).
 8. The method as setforth in claim 7, wherein the hydraulic fluid is maintained by a heatexchanger at a temperature that corresponds to the mean temperature ofthe working fluid in the pneumatic-hydraulic converter (8).
 9. Themethod as set forth in claim 8, comprising directing the working fluidfrom the pneumatic-hydraulic converter (8) through a second heatexchanger (16).
 10. The method as set forth in claim 9, comprisingexpanding the working fluid coming from the pneumatic-hydraulicconverter (8) to an expansion pressure that is lower than the firstpressure in the storage reservoir (1) and compressing the working fluidto the first pressure thereafter.
 11. An apparatus for convertingthermal energy to mechanical work, said apparatus comprising a storagereservoir (1), a work tank (3), and a working machine (9) for convertinghydraulic work into mechanical work, wherein the work tank (3)communicates with a first heat exchanger (5) in order to heat theworking fluid, that the work tank (3) is further connected to apneumatic-hydraulic converter (8) that transfers the pressure of theworking fluid to a hydraulic fluid and that there is provided arecirculation line for recirculating the working fluid from thepneumatic-hydraulic converter (8) into the storage reservoir (1). 12.The apparatus as set forth in claim 11, including a feed pump (2) forpumping the working fluid from the storage reservoir (1) into the worktank (3).
 13. The apparatus as set forth in claim 12, wherein the firstheat exchanger (5) is mounted in the work tank (3).
 14. The apparatus asset forth in claim 13, wherein the working machine (9) is a hydraulicmotor.
 15. The apparatus as set forth in claim 14, wherein thepneumatic-hydraulic converter (8) a bladder accumulator.
 16. Theapparatus as set forth in claim 15, including a second heat exchanger(16) interposed between the pneumatic-hydraulic converter (8) and thestorage reservoir (1).
 17. The apparatus as set forth in claim 16,wherein the second heat exchanger (16) is a condenser.
 18. The apparatusas set forth in claim 17, including a booster pump downstream of thesecond heat exchanger (16).
 19. The apparatus as set forth in claim 18,wherein the work tank (3) is an evaporator.
 20. The apparatus as setforth in claim 19, including a third heat exchanger (11) in the circuitof the hydraulic fluid.
 21. The apparatus as set forth in claim 20,including an internal combustion engine having a cooling system thatcommunicates with the work tank (3).
 22. The apparatus as set forth inclaim 21, including a plurality of work tanks (3) and ofpneumatic-hydraulic converters (8) connected in parallel.